The present invention relates to a method for producing polycarbonate employing ester interchange.
More specifically, the present invention relates to a method for producing a high molecular weight aromatic polycarbonate with improved color from an aromatic diol and a carbonate diester.
Aromatic polycarbonates have recently been employed as engineering plastics with excellent mechanical properties such as impact strength as well as heat resistance and transparency.
A known method for producing such aromatic polycarbonates, the so-called phosgene process, involves reacting an aromatic diol such as bisphenol with phosgene via interfacial polycondensation. However, a great number of problems have been observed with regard to this process including the requirement of using extremely toxic phosgene, the disposal of a great amount of sodium chloride as a byproduct and health and environmental concerns for the methylene chloride generally employed as the reaction solvent in the process.
The so-called melt process, or non-phosgene process, has also been known for obtaining an aromatic polycarbonate via ester interchange between an aromatic diol and a carbonate diester. The non-phosgene method is regarded as advantageous in that it avoids the various problems of the phosgene process described above and in that the aromatic polycarbonate can be produced more economically.
However, in order to obtain a high molecular weight polycarbonate with excellent mechanical properties in accordance with the non-phosgene method using bisphenol A and diphenyl carbonate, it is required to distill phenol and diphenyl carbonate from a melt of a higher viscosity. Thus, the resulting polycarbonate is generally exposed to vacuum at a temperature of from 250.degree. to 330.degree. C. for a long period of time. The problem arises that the resulting polymer is yellow colored. Employing the non-phosgene method, products of a balanced quality between color value and molecular weight are generally difficult to produce.
With the object of improving the thermal stability and color value of aromatic polycarbonates, a variety of phosphorous compounds have been conventionally used in various methods.
With the object of improving the thermal stability, color value or resistance to hydrolysis of an aromatic polycarbonate during molding and kneading, one known method comprises adding various phosphorous compounds to the polycarbonate during molding and kneading, Japanese Patent Unexamined Publication Nos. 50-51154 and 52-147655, U.S. Pat. Nos. 3,305,520 and 3,404,122).
These methods, however, will not prevent the deterioration in color occurring during polymerization since the phosphorous compounds are added after the polymerization of the aromatic polycarbonates.
It is also known to use various phosphorous compounds in the polycondensation reaction for ester interchange. Illustrative of methods of this type are:
(a) use of a combination of a phosphonite and a hindered phenol in the presence of a quaternary ammonium compound (Japanese Patent Unexamined Publication No. 61-151236);
(b) use of diphenyl phosphonate in the presence of KBH.sub.4 catalyst (Japanese Patent Unexamined Publication Nos. 62-158719 and 62-169821);
(c) effecting melt polycondensation with an electron donor phosphorous compound as catalyst (Japanese Patent Unexamined Publication No. 3-54223); and,
(d) effecting melt polycondensation with ester interchange between an alicyclic diol and a carbonate diester (Japanese Patent Unexamined Publication No. 2-180954).
Employing these methods, however, it is difficult to obtain a high-molecular weight aromatic polycarbonate with excellent color.